Iridium Stabilizes Ceramic Titanium Oxynitride Support for Oxygen Evolution Reaction

Decreasing iridium loading in the electrocatalyst presents a crucial challenge in the implementation of proton exchange membrane (PEM) electrolyzers. In this respect, fine dispersion of Ir on electrically conductive ceramic supports is a promising strategy. However, the supporting material needs to meet the demanding requirements such as structural stability and electrical conductivity under harsh oxygen evolution reaction (OER) conditions. Herein, nanotubular titanium oxynitride (TiON) is studied as a support for iridium nanoparticles. Atomically resolved structural and compositional transformations of TiON during OER were followed using a task-specific advanced characterization platform. This combined the electrochemical treatment under floating electrode configuration and identical location transmission electron microscopy (IL-TEM) analysis of an in-house-prepared Ir-TiON TEM grid. Exhaustive characterization, supported by density functional theory (DFT) calculations, demonstrates and confirms that both the Ir nanoparticles and single atoms induce a stabilizing effect on the ceramic support via marked suppression of the oxidation tendency of TiON under OER conditions.

: Statistical analysis of Ir nanoparticles before and after EC-P and after EC-CV for TiON-Ir sample. a) Particle size distribution, b) particle dispersion and c) particle circularity.

S1. Estimation of TiO2 layer thickness after degradation of TiON and TiON-Ir
One potentially viable way of estimating the TiO2 layer thickness evolved under electrochemical conditions is from a linear regression extrapolation. If we extrapolate the linear regression formula of TiON after the electrochemical perturbation (Figure 2 after degradation equation) to (N/O ratio)=0, we get a value of thk(TiO2)=2.8 nm, where thk(TiO2) represents the thickness of TiO2 layer without nitrogen. However, one cannot use this method to estimate the thickness of TiO2 layer for the TiON-Ir analogues after electrochemical perturbation because of the low R 2 value of the linear regression model for N/O ratio vs. thickness (0.0143, 0.0598, 0.3404, Figure 3). Hence, an alternative approach is needed for TiON-Ir that can also be used for TiON to compare both results. Initially, one can assume that the two average N/O ratios of TiON and TiON core (before and after electrochemical perturbation, respectively) are the same (Figure 2a). Then, one asks themselves, how thick does the TiO2 layer after the electrochemical perturbation need to be in order to obtain the same N/O ratio as in every data point in the graph in Figure 2 (after degradation data points). This estimation method assumes the formation of a pure TiO2 layer as the only oxidation mechanism.
We express n(O) TiO 2 from equation (S3) and get We calculate the molar fraction of oxygen and titanium coming from TiO2 in the layer after degradation We can assume that the TiO2 layer has the following titanium and oxygen ratio (Ti) TiO Table S2 suggests that a TiO2 layer has formed during both degradation protocols. Interestingly, for the TiON sample, the slight decrease of the average (N+O)/Ti ratio found in Table S2 suggests that no TiO2 layer has formed during the degradation protocol but instead the TiON structure remained and N atoms were exchanged for O atoms.  Table S3. Carbon was assumed to be due to contamination, and it was not taken into account although it was detected. Also, elements Si and K were detected in the amount of a few at. %, but they were not considered for quantification. After nitridation, N is introduced into the TiO2 structure resulting in 51 at. % of O, 23 at. % of N, and 26 at. % of Ti (approximately Ti1N1O2). The excess O may be contributed to surface contamination, which is detected due to high surface sensitivity of the XPS method.
High-energy resolution XPS spectra were acquired to get insight into the surface chemistry after different sample treatments. Figure S3 shows XPS spectra Ti 2p and Ir 4f after Ir deposition, EC-P, and EC-CV for TiON-Ir sample. Ti 2p spectra consist of the Ti 2p3/2 and Ti 2p1/2 peaks separated by 5.9 eV. After nitridation, N 1s peak at 396.0 eV appears, related to the nitride or oxynitride formation. In addition to Ti 2p3/2 peak at 458. eV, and Ir 4f5/2 separated for 3.0 eV to the higher binding energy. After Ir deposition, the Ir 4f7/2 spectrum was deconvoluted in a peak at 61.4 eV related to the Ir(0) metallic state and the second peak at 62.6 eV, which is related to Ir-oxide in the Ir(4+) state. 2 Their intensities show that after Ir deposition, about 60 % of the Ir atoms are in a metallic state, and about 40 % are in Ir(4+). We should note that the Ir 4f spectrum overlaps with the Ti 3s spectrum at 60 eV, making it difficult to quantify Ir presence.
After EC-P, the Ir 4f spectrum still shows the presence of two oxidation states of Ir, but the Ir(4+) state is 70 %, and Ir(0) presents 30 % of total Ir atoms ( Figure S3b). After EC-CV, all Ir atoms were found in the Ir(4+) oxidation state ( Figure S3b). The total concentration of Ir on TiON-Ir after EC-P is similar as after deposition, i.e., 1.5 ± 0.3 at. %, and after EC-CV, there is 1.0 ± 0.3 at. % of Ir. We observed that Ir concentration is decreased after EC-CV.
Another change observed by XPS after EC-P and EC-CV is that the concentration of N decreased, and O concentration increased (Table S3)

S3. Raman spectroscopy
Raman spectroscopy was used as another technique to evaluate structural properties of TiON, TiON-Ir, after EC-P, and EC-CV samples. The characterization was performed in the ex-situ mode, i.e., before and after the electrochemical treatment ( Figures S5 and S6). For each kind of sample, the spectra were measured on 3 different sites and, on each of them, by sequentially applying an increasing laser power. Such a measurement protocol enables estimations of the samples' stability and an example of such a measurement procedure is depicted in Figure S5. The inset clearly shows how the intensity in the as-measured spectra increases as a function of the increasing laser power. Of course, due to the high laser power some band changes can occur. It is thus self-evident that only Raman spectra recorded with the low laser powers can be used to interpret the structure. With the aim to enable comparison of the spectra, they were adapted to more similar intensities ( Figure S6). The factors that were used to adapt the spectral intensities are not essential for a qualitative comparison. The Raman spectra of all measured sites are shown in Figure S6 in the spectral range 20 cm -1 to 1100 cm -1 . Evidently, the spectra started to change gradually when the laser power of 3.4 mW was employed, but the extent of changes differed.
The spectra of the TiON sample (0.6 mW) reveal a broad group of bands in the investigated structural range (Figure S6a (Figure S6 A). Obviously, a significant transformation to anatase occurred (Figure S6 B). The characteristic anatase triple bands also started to become visible. The B3 site, however, showed slower transformation.  After EC-P: Interestingly, after the electrochemical treatment (Figure S6 C), the shape of the spectra recorded at 0.6 mW (green spectra) resembles the spectra of the TiON sample (Figure S6 A), with the bands appearing at 203 cm −1 , 363 cm −1 , and 515 cm −1 (sh), 579 cm −1 , marked with vertical green dashed lines. With increasing laser power, the stability of this sample is considerably improved with regard to the TiON-Ir sample (Figure S6 B).
After EC-CV: The spectra recorded after EC-CV revealed that these samples ( Figure   S6 D) were similarly stable as the TiON samples in Figure S6 A. Moreover, these spectra confirmed that two different types of sites exist. The spectra from the D1 site represent the N-rich site with bands more similar to the TiN spectrum, 3,5 the doublet band at 217 cm −1 and 321 cm −1 , and a broad band at 602 cm −1 with a shoulder at 529 cm −1 , marked with vertical solid red lines in Figure S6 D. The spectra from the other two sites (D2 and D3) resemble the spectra of the TiON samples (Figure S6 A).
Evidently, the stability of EC-CV sample under the increasing laser power was comparable to that of the TiON samples.
Raman measurements clearly showed that the TiON and EC-CV samples were the most stable under the increasing laser power. The EC-P sample was somewhat less stable, while the TiON-Ir sample was more susceptible to transformation into anatase.
The increased stability of the electrochemically treated EC-CV and EC-P samples during Raman measurement (Figure S6 C,D) supports our claim that the electrochemically formed Ir SAs are associated with the increased stability.

S4. Iridium redox charge-normalized OER performance
The OER polarization curves obtained before/after sequential electrochemical treatments (i.e. Before EC-P, After EC-P and After EC-CV) were normalized also per characteristic Ir(III/IV) redox charge in the potential window of 0.6 -1.1 V, respectively.
The charge was obtained during fast cyclic voltammetry in between treatments ( Figure S7a). The most significant charge increase occurs after the EC-CV protocol ( Figure S7b). Interestingly though are the following two facts. Firstly, the corresponding OER performance trend (Figure S7c,d)   . Either anodic or cathodic branch (0.6 -1.1 V) was used to calculate the Ir charge. b) Iridium redox charge obtained from a) before or after EC-P and EC-CV electrochemical biasing, respectively. c) Iridium redox charge-normalized OER polarization curves measured either before or after EC-P and EC-CV electrochemical biasing, respectively. d) Tafel plots of OER polarization curves (constructed from c).

S5. Potentiostatic treatment of TiON, TiON-Ir and EELS analysis
A separate TiON-Ir sample (S-TiON-Ir) was prepared and electrochemically treated under identical conditions as TiON sample (1.6 V for 30 min, from here on refered to as EC-stat). Before and after EC-stat an EELS analysis was performed, however under non-identical location mode (Table S4). The EELS results indicate that EC-stat induces similar trends as EC-P and EC-CV excluding the possibility of IL-EELS results being misinterpreted due to a non-identical electrochemical protocol. However, as evident from the corresponding electrochemical characterization, a static protocol did not enable sufficient removal of oxygen bubbles. More specifically, current response is gradually decreasing and eventually approaches bare TiON currents ( Figure S8).